This invention relates to a method for the prediction of performance of a centrifugal pump, and more particularly to a method for the accurate prediction of performance of a centrifugal pump with a thrust balance mechanism by use of a quasi-three-dimensional flow analysis.
A multistage centrifugal pump with an inducer has been widely used for transporting certain dangerous liquids such as LPG, LNG, liquid hydrogen and liquid oxygen. Since the reliability and safety of this pump depends on an axial thrust balance and a leakage seal, it is essential to use an axial thrust mechanism for a dip-type pump or a scanned motor pump, and otherwise to use a complicated shaft seal gear.
It is difficult to use the above-mentioned dangerous liquids to test the performance of this pump. Therefore, a substitute liquid is generally used and actual pump performance is determined by the use of a performance conversion chart. Generally, the pump performance indicated by a non-dimensional amount of liquid is identical among all kinds of liquids, except for the influence of viscosity of the fluid.
For instance, a formula proposed by Moody, Hutton and Ackeret has already been widely used as an extended formula for calculating the influence of the fluid viscosity in a turbine on the non-dimensional performance characteristics. Another well-known method for determining performance conversions for turbines more accurately than that proposed by Moody et al is the JSME standard S-008 issued in 1989.
In a conventional performance prediction analysis of a centrifugal pump or turbine, various losses, which are obtainable by one dimensional analysis, are subtracted from a theoretical pump head to analyze the performance of an impeller. Further, in an analysis of a balance mechanism, such as an analysis of the inside flow, it is assumed that the fluid between a rotary wall and a stationary wall shows a forced vortex motion at half of the velocity of the rotary wall so that friction power and the axial thrust can be analyzed.
In this kind of pump, however, there is less available data to compare a model test with the actual performance. Thus, it is difficult to evaluate the accuracy of the performance conversion.
Conventional performance prediction analysis also has other drawbacks. Due to the use of one dimensional analysis, it is difficult to accurately analyze the flow of the fluid inside the impeller. Thus, there is a large difference between the predicted performance and the observed performance at non-design points. This happens particularly in a low flow-rate region, even if it is possible to predict the performance near the design point. Further, it is difficult to have an accurate grasp of the fluid flow or inertial flow of the fluid in the balance mechanism. This creates a difficulty in evaluating variations of the axial thrust. Further, the leakage characteristics as a function of variations of clearance and the viscosity of the fluid cannot be accurately evaluated.
Further, it is difficult to conduct simultaneous analysis of the impeller and the balance mechanism, both of which are associated with each other, for determining an operating point or a position of the thrust balance mechanism.